CI: Make version finding more flexible

Use repository name rather than hard-coding MetPy, which allows this
action to be re-used in Siphon. Also print out the value since this is
often needed in debugging.

.github/actions/build-docs/action.yml

@@ -27,7 +27,11 @@ runs:
     - name: Set doc version
       id: docversion
       shell: bash -l {0}
-      run: echo "doc_version=$(python -c 'import metpy,re; print(re.search(r"(\d+\.\d+)", metpy.__version__)[0])')" >> $GITHUB_OUTPUT
+      run: echo "doc_version=$(python -c 'import importlib.metadata,os,re; print(re.search(r"(\d+\.\d+)", importlib.metadata.distribution(os.environ["GITHUB_REPOSITORY"].split("/")[-1].lower()).version)[0])')" >> $GITHUB_OUTPUT
+
+    - name: Show doc version
+      shell: bash -l {0}
+      run: echo Building docs for version ${{ steps.docversion.outputs.doc_version }}
 
     - name: Build docs
       shell: bash -l {0}